There is an ever increasing need for effective safety closures such as closures which prevent accidental opening of a container as by children. A number of approaches in the prior art has been taken. Exemplary of these are U.S. Pat. Nos. 3,892,326, 2,980,275; 3,019,931 and 3,567,057. An arrangement which has met with warm acceptance is disclosed in U.S. Pat. No. 3,744,655. In that patent the neck is provided with a locking ring which has thereon a hinged tab carrying a locking tooth for engagement with notches in the cap. To disengage the locking tooth the tab is depressed so that the cap can be unscrewed in a conventional manner. A further arrangement which incorporates the principles of U.S. Pat. No. 3,744,655 is to provide a pair of oppositely disposed tabs on the locking ring with the locking teeth, however, being not quite 180.degree. apart. This further arrangement has the obvious deficiency in its removal operation since it requires the locking teeth on both tabs to be manipulated out of engagement with the cap notches while the cap itself is being unscrewed.
While the hinged tab arrangement of U.S. Pat. No. 3,744,655 has been successful, it is possible that occasionally during assembly of the cap on the ring, the locking tooth will not be exactly registered in the desired position with respect to the notches in the cap. For example the cap notches contain slanted faces and the notches terminate in an apex. The ring locking tooth is similarly shaped and ideally should fit within the notch at the apex. If, however, during assembly the tooth abuts against the slanted notch wall rather than fitting in the apex there is a tendency to urge the hinged tab downwardly with a notch apex. Where on occasion the locking tooth is not positioned in its ideal location with respect to the cap notches, this improper positioning may be maintained over a long period of time such as while the container is in transit and is being stored prior to sale. When the container is subsequently used by removing the cap, the tab might tend to return only to the position it had during the long period of storage whereupon it might not register completely with the apex of the notch even if disposed at the apex. Under such conditions the system would not function as an effective safety closure. This problem can be avoided by using a material which has proper "memory" characteristics. For example U.S. Pat. No. 3,744,655 discloses the use of a polycarbonate or Lexan (Reg. T.M. of G.E. Co.) as a suitable material. A plastic such as Lexan inherently retains it memory that is even if the tab should be depressed over a prolonged period such as by improper registration of the locking tooth in a cap notch, the tab would tend to return completely to its normal intended condition rather than be deformed in the improper position.
While Lexan has proven to be a successful material for use with closures of the above patent, materials such as Lexan have the disadvantage of being relatively expensive. It would, therefore, be desirable to provide a safety closure system having the advantages of the above noted patent yet capable of using more economical materials such as, for example, lower cost polypropylenes.